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          java Socket初探—服务器与局域网
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        <p>自己很久以来一直想做个局域网内传输文件的程序，一直没有动手。这次学习了一下要用到的局域网内的socket通信，在此做个记录。</p>
<h2 id="局域网内的socket通信"><a href="#局域网内的socket通信" class="headerlink" title="局域网内的socket通信"></a>局域网内的socket通信</h2><p>并无特别之处，只是需要使用内网IP来建立连接。服务端开启一个ServerSocket，监听特定的端口，获取输出流向客户端写入数据。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span> &#123;</span><br><span class="line">    <span class="type">ServerSocket</span> <span class="variable">serverSocket</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">ServerSocket</span>(<span class="number">9998</span>);</span><br><span class="line">    <span class="comment">// 也可以绑定监听的IP，应为内网IP</span></span><br><span class="line">    <span class="comment">//ServerSocket serverSocket = new ServerSocket(9998, 50, InetAddress.getByName(&quot;xx.xx.xx.xx&quot;)); // xx.xx.xx.xx 为本机的内网IP</span></span><br><span class="line">    <span class="keyword">while</span> (<span class="literal">true</span>) &#123;</span><br><span class="line">        <span class="type">Socket</span> <span class="variable">s</span> <span class="operator">=</span> serverSocket.accept();</span><br><span class="line">        <span class="type">PrintWriter</span> <span class="variable">pw</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">PrintWriter</span>(s.getOutputStream());</span><br><span class="line">        pw.println(<span class="string">&quot;hello world&quot;</span>);</span><br><span class="line">        pw.flush();</span><br><span class="line">    &#125;</span><br><span class="line">&#125; <span class="keyword">catch</span> (IOException e) &#123;</span><br><span class="line">    e.printStackTrace();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>客户端向服务端的”<em>内网IP:指定端口</em>“发送连接请求，读写数据，</p>
 <span id="more"></span>

<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span> &#123;</span><br><span class="line">    <span class="type">Socket</span> <span class="variable">s</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Socket</span>();</span><br><span class="line">    s.connect(<span class="keyword">new</span> <span class="title class_">InetSocketAddress</span>(<span class="string">&quot;xx.xx.xx.xx&quot;</span>, <span class="number">9998</span>)); <span class="comment">// xx.xx.xx.xx 为服务端的内网IP</span></span><br><span class="line">    <span class="type">BufferedInputStream</span> <span class="variable">in</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">BufferedInputStream</span>(s.getInputStream());</span><br><span class="line"></span><br><span class="line">    <span class="type">int</span> <span class="variable">len</span> <span class="operator">=</span> <span class="number">0</span>;</span><br><span class="line">    <span class="type">byte</span>[] bytes = <span class="keyword">new</span> <span class="title class_">byte</span>[<span class="number">1024</span>];</span><br><span class="line">    <span class="keyword">while</span> ((len = in.read(bytes)) != -<span class="number">1</span>) &#123;</span><br><span class="line">        System.out.println(<span class="keyword">new</span> <span class="title class_">String</span>(bytes, <span class="number">0</span>, len));</span><br><span class="line">    &#125;</span><br><span class="line">&#125; <span class="keyword">catch</span> (IOException e) &#123;</span><br><span class="line">    e.printStackTrace();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>另外，需要设置防火墙把使用的端口开放才可以成功。</p>
<p>以上的例子是客户端已知服务端内网IP下进行的，而想做的局域网内文件传输的程序如果还需要用户手动输入IP就贻笑大方了。为了可以在不知道服务端IP情况下开始建立连接，我们可以在客户端发送一个UDP广播。</p>
<h2 id="UDP广播"><a href="#UDP广播" class="headerlink" title="UDP广播"></a>UDP广播</h2><p>UDP 的主要实现在DatagramSocket类，在服务端监听指定端口，</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">DatagramSocket</span> <span class="variable">ds</span> <span class="operator">=</span> <span class="literal">null</span>; <span class="comment">// 创建接收数据报套接字并将其绑定到本地主机上的指定端口</span></span><br><span class="line"><span class="keyword">try</span> &#123;</span><br><span class="line">    ds = <span class="keyword">new</span> <span class="title class_">DatagramSocket</span>(<span class="number">9998</span>); <span class="comment">// 监听9998端口</span></span><br><span class="line">    <span class="type">byte</span>[] buf = <span class="keyword">new</span> <span class="title class_">byte</span>[<span class="number">1024</span>];</span><br><span class="line">    <span class="type">DatagramPacket</span> <span class="variable">dp</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">DatagramPacket</span>(buf, <span class="number">1024</span>);</span><br><span class="line">    ds.receive(dp); <span class="comment">// 接收报文，此方法将会在接收到一个报文前一直堵塞</span></span><br><span class="line">    <span class="type">String</span> <span class="variable">data</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">String</span>(dp.getData(), <span class="number">0</span>, dp.getLength());</span><br><span class="line">    System.out.println(data);</span><br><span class="line">    ds.close();</span><br><span class="line">&#125; <span class="keyword">catch</span> (SocketException e) &#123;</span><br><span class="line">    e.printStackTrace();</span><br><span class="line">&#125; <span class="keyword">catch</span> (IOException e) &#123;</span><br><span class="line">    e.printStackTrace();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>报文类DatagramPacket对象还存储了发送方的IP地址InetAddress address和端口port，可以使用getAddress()和getPort()得到。</p>
<p>客户端则是发送一个广播，</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">DatagramSocket</span> <span class="variable">s</span> <span class="operator">=</span> <span class="literal">null</span>;</span><br><span class="line"><span class="keyword">try</span> &#123;</span><br><span class="line">    s = <span class="keyword">new</span> <span class="title class_">DatagramSocket</span>();</span><br><span class="line">    <span class="type">String</span> <span class="variable">str</span> <span class="operator">=</span> <span class="string">&quot;hello, I&#x27;m client&quot;</span>;</span><br><span class="line">    <span class="type">DatagramPacket</span> <span class="variable">dp</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">DatagramPacket</span>(str.getBytes(),</span><br><span class="line">            str.getBytes().length,</span><br><span class="line">            InetAddress.getByName(<span class="string">&quot;255.255.255.255&quot;</span>), <span class="number">9998</span>);</span><br><span class="line"></span><br><span class="line">    s.send(dp);</span><br><span class="line">&#125; <span class="keyword">catch</span> (SocketException e) &#123;</span><br><span class="line">    e.printStackTrace();</span><br><span class="line">&#125; <span class="keyword">catch</span> (UnknownHostException e) &#123;</span><br><span class="line">    e.printStackTrace();</span><br><span class="line">&#125; <span class="keyword">catch</span> (IOException e) &#123;</span><br><span class="line">    e.printStackTrace();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

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